1. Field of the Invention
This invention relates to microelectromechanical devices, and more particularly, to the arrangement and number of contact structures and support beams within a plate-based microelectromechanical device.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Microelectromechanical devices, or devices made using microelectromechanical systems (MEMS) technology, are of interest in part because of their potential for allowing integration of high-quality devices with circuits formed using integrated circuit (IC) technology. As compared to transistor switches formed with conventional IC technology, for example, microelectromechanical contact switches may exhibit lower losses and a higher ratio of off-impedance to on-impedance. MEMS switch designs generally use an actuation voltage to close the switch, and typically rely on the spring force in the beam or plate to open the switch when the applied voltage is removed. In opening the switch, the spring force of the beam or plate must typically counteract what is often called “stiction.” Stiction refers to various forces tending to make two surfaces stick together such as van der Waals forces, surface tension caused by moisture between the surfaces, and/or bonding between the surfaces (e.g., through metallic diffusion). Consequently, actuating a switch at a relatively low voltage tends to make the switch harder to open, resulting in a switch which may not open reliably (or at all).
For this reason, it is often desirable within MEMS switches to apply high actuation voltages, such as on the order of 50 volts or more, such that a complementary spring force sufficient to open the switch is stored within the switch. Such relatively high actuation voltages, however, often require voltage translation circuits when used with transistor switches, increasing the complexity of the circuit. In addition, relatively high actuation voltages increase the force attracting the electrodes of a MEMS switch. In some cases, the actuation voltages may be high enough to cause the electrodes to contact, causing the device to malfunction. As such, it is often desirable to optimize actuation voltages of MEMS switches such that the switch can reliably open and close but the electrodes can be prevented from contacting.
MEMS switch designs are often characterized by the form of their moveable component/s. For example, a cantilever-based MEMS switch includes a moveable beam supported at one end and free at another. In contrast, strap-based MEMS switches include a moveable beam supported at both ends. A third class of MEMS switches is diaphragm-based structures in which a membrane is supported around most or all of its perimeter. In some MEMS switches, a moveable plate is used instead of a cantilever beam, strap beam, or diaphragm membrane. In some embodiments, the moveable plate may be supported by support structures arranged at each of the four corners of the plate (i.e., when a square or rectangular plate is employed). The support structures of plate-based MEMS switches differ from support structures used for cantilever-based, strap-based and diaphragm-based MEMS switches in that they are configured to twist and bend such that the entire plate may move relative to a fixed electrode. Such an adaptation of support structures, however, may cause plate-based MEMS switches to be more susceptible to having electrodes collapse onto each other, particularly at high actuation voltages. In addition, high actuation voltages may cause the plate itself to bend such that a portion of the plate contacts the underlying gate electrode, particularly if the plate is not evenly supported by the structures. Consequently, the tolerance of actuation voltages for plate-based MEMS switches are often small or cannot be effectively optimized to allow the switches to be reliably opened and closed while simultaneously preventing the actuation electrodes of the switches from contacting one another.
It would, therefore, be desirable to develop a plate-based MEMS switch which relaxes the aforementioned constraints imposed by the use of high actuation voltages, namely opening and closing reliability and the prevention of collapsing electrodes.